1. Field of the Invention
The present invention generally relates to solar cells, and more particularly, to an improved fine geometry solar cell wherein the contact area between the fine metallic fingers of the photo current collecting electrode and the light-incident surface of the solar cell are minimized.
Description of the Prior Art
The use of photovoltaic devices, commonly known as solar cells, which convert light energy to useful electrical energy is well known. Light entering these solar cells is absorbed, thereby generating Electron-hole pairs which are then spatially separated by the electric field produced by the solar cell junction and are collected at respective top and bottom surfaces of the solar cell. For example, in an n-p type solar cell electrons will travel to the top surface where they will then be collected by a metallic grid positioned thereon. The metallic grid may typically comprise a number of metallic fingers separated along the top surface by a relatively large distance and connected to each other by a common bus bar. The electrons will travel either directly to the metallic fingers or approach the top surface between the fingers and then travel along the surface of the solar cell until they can be collected by one of the fingers. Holes, on the other hand, will travel to the bottom surface of the solar cell where they may be collected by a metallic sheet covering the entire bottom surface.
A major improvement to solar cell design is described in U.S. Pat. No. 3,811,954 issued to Joseph Lindmayer and assigned to Communications Satellite Corporation. The improved solar cell described in the Lindmayer patent has the advantage of improving the efficiency of solar cells in the short wavelength, i.e., blue-violet portion of the spectrum corresponding to 0.3-0.5 microns thereby sharply increasing output power. This was accomplished basically by reducing the junction depth and providing a very fine geometric pattern metallic electrode on the light-incident surface of the solar cell. The junction depth is typically between 500 A to 2,000 A, and the metallic fingers of the fine geometric electrode are typically spaced on the order of only a few hundredths of a centimeter. The resultant fine geometric pattern electrode generally covers and is in ohmic contact with between 5 to 10% of the surface area of the solar cell and prevents the covered area from converting incident photons to electrical energy. The surface area which is not in contact with the electrode is covered by a metal oxide anti-reflective coating.
Reverse currents in silicon solar cells comprise the flow of electrons and holes to the junction where they recombine. This current degrades the performance of the cell, i.e. the I-V characteristic, by opposing a part of the primary current flow across the diode. Reverse currents which typically have been considered in solar cell models result from the bulk and space charge regions of semiconductors. Another source of reverse current is the top surface layer where impurities are centers of recombination as well as sources of thermally generated electron-hole pairs. However, this current is generally neglected in the standard solar cell since there is a high concentration of impurities which act to produce a minimum of electron-hole pairs that recombine at the junction. Furthermore, a thick "dead layer" isolates the surface generation centers in the top layer of the solar cell and also any effect from the ohmic contact with the metallic electrode. In the improved solar cell according to the Lindmayer patent, the shallow junction results in a minimization of the dead layer which exists at the light-incident surface of the normal silicon cell. Furthermore, the density of impurities diffused into the top layer of the cell is descreased and provides sources for a significant reverse current flow. The ohmic contact formed between the metallic electrode and the top layer of the silicon solar cell causes thermal generation of currents to occur near the surface of the solar cell. The metallic contact acts as a large sink for recombination and also as a large source for generation of electron-hole pairs.